Further follow-up.

If you're having twins, be advised that a side-by-side travel system does not accommodate two infant carriers.  At best, you can place one infant carrier on the left side, and remove the other baby from the other infant carrier and place her into the stroller itself.

We think this is because it's impossible for two infant carriers to go side-by-side through a standard doorway, so the stroller designers just gave up trying to get both to fit on the same side-by-side stroller.  Forewarned is forearmed, or in this case, just "less disappointed."